A variable name should not start with `$`

[saig0]

Describe the bug
Currently, a variable name can start with a $ sign. According to the spec, $ is not a valid start of a variable name.

Related to bpmn-io/feel-editor#28.

To Reproduce
Steps to reproduce the behavior:

1. Evaluate the expression $a with a context {"$a": 1}
2. Verify that the expression is evaluated successfully

Expected behavior
The $ sign should not be a valid start of a variable name. As a result, an expression should not evaluate successfully (i.e. should not parse successfully) if it contains a variable name that starts with a $ sign.

Environment

• FEEL engine version: 1.15
• Affects:
  • Camunda Automation Platform 7: [7.x]
  • Zeebe broker: [0.x]